In any situation where power or torque is transmitted from one rotatable shaft to another, the shafts should ideally be in precise alignment. Proper alignment of the shafts reduces the performance requirements imposed on the coupling mechanism and optimizes the operation of the shafts and the machinery to which each shaft is connected. Although the two shafts may be precisely aligned when first installed, it is practically impossible to maintain such an alignment. This is particularly true when the shafts are incorporated in a moving structure such as a motor vehicle or an aircraft. Flexing of the body of the motor vehicle or aircraft may cause the mounting points or journals for the shafts to move relative to one another. Vibrations due to mass imbalances may cause the shafts to move out of alignment, just as may wearing of the journal bearings for the shafts and wearing of the shafts themselves. In many installations, therefore, the coupling or joint that connects the two shafts must be constructed to accommodate significant angular misalignments of the shafts.
A Hooke's or cardan type joint or coupling will permit the transmission of torque or power from one rotatable shaft to another despite angular misalignment of the shafts. Nonetheless, as larger and more powerful engines have been developed to turn shafts at higher rotational speeds, the loads to which shafts may be subjected at start up of an engine or during operation, due to variations in rotational speed, have substantially increased. To protect drive shafts and the connections between shafts against such shock and vibration loads, many universal couplings of the Hooke's type incorporate resilient bushings. Thus, as is shown in Moulton et al U.S. Pat. No. 2,975,621, a Hooke's type coupling may include a four armed spider with an annular bushing of elastomeric material encircling each arm of the spider. Each pair of opposed arms of the spider is connected to a different shaft.
One disadvantage of a Hooke's type coupling is that the coupling itself has essentially no capability for permitting relative axial movement between two shafts (i.e., relative movement between the shafts along or parallel to a common axis). To provide for accommodation of relative axial movements between two shafts joined by a coupling, while also providing torsional resilience and protection against shock and vibration loads, a different type of coupling may be employed. In such a coupling, a central hub is joined to a rigid outer ring by a ring-like mass of elastomer. The elastomer is bonded at least to the outer circumference of the hub, and preferably also to the inner circumferential surface of the outer ring. The hub of the coupling is connected to one shaft, while the outer ring of the coupling is joined to the second shaft. Power or torque is transmitted from one shaft to the other through torsional stressing of the elastomer in the coupling. When sufficient clearance is provided between the adjacent ends of the two shafts being joined by the coupling, relative axial movements between the shafts can be accommodated through shearing deflection of the elastomer in the coupling. The elastomer may also be deflected to permit angular misalignment between the shafts. A basic form of such a flexible or elastomeric ring type coupling, in which the hub and the outer ring are both cylindrical in shape, is illustrated in FIG. 5 of Sampson U.S. Pat. No. 2,154,077.
Although an elastomeric ring type coupling can accommodate relative axial movements between two shafts that are joined by the coupling, the motion accommodation capability can pose additional problems. For example, in order for a coupling to be able to accommodate relative axial movement, there must be sufficient clearance between the adjacent ends of the shafts. There must also be a radial thickness of elastomer large enough to permit the elastomer to be deflected in shear a distance far enough to accommodate the maximum relative axial movement that is expected between the shafts. Unfortunately, in providing a sufficient radial thickness of elastomer to accommodate the expected axial movement between the shafts, the drive coupling may be rendered unacceptably soft in the radial direction. In other words, the hub, as well as the shaft joined to the hub, may be moveable too easily in radial directions relative to the outer ring and to the shaft attached to the outer ring. To overcome unacceptable radial softness of an elastomeric ring type coupling, while still providing the necessary capability to permit axial movement, one or more concentric rings or shims of substantially inextensible material may be embedded in the elastomer at various radial distances from the hub. The addition of the rings or shims will reduce the ability of the elastomer to bulge in response to radial loads and thereby decrease the deflection of which the elastomer is capable in response to any given radial load. At the same time, the ability of the elastomer to deflect in shear under a given load along the longitudinal axis of the coupling, and thus the ability of the coupling to accommodate relative axial movements between the two shafts that it is connecting, will be substantially unaffected. A basic laminated ring type coupling such as has just been discussed is described and illustrated in Julien U.S. Pat. No. 2,187,706. A similar drive coupling is described and illustrated in Swiss Pat. No. 216,216, while Wilhaber U.S. Pat. No. 2,760,359 describes and illustrates a Hooke's type coupling with laminated elastomeric bushings.
As previously mentioned, an elastomeric ring type coupling can accommodate angular misalignment, as well as relative axial movements, between two shafts that are joined by the coupling. To permit angular misalignments, the hub of the coupling must cock or tilt with respect to the outer ring of the coupling about an axis that is generally perpendicular to the longitudinal axis of the coupling. If the hub and the outer ring are both cylindrical in shape, such a cocking or tilting motion between the hub and the outer ring can only be permitted through compression loading of at least a portion of the elastomer. Loading the elastomer in compression will tend to impose some limitations on the degree of angular misalignment that can be accommodated and will also produce a relatively high cocking stiffness for the coupling. If, however, the outer circumferential surface of the hub of such a ring-type coupling, together with the inner circumferential surface of the outer ring of such a coupling, is shaped as a portion of a sphere, angular misalignment between the shafts joined by the coupling will cause the elastomer in the coupling to be stressed primarily in shear. Because an elastomer is naturally at least three times as stiff in compression as it is in shear, the change to a spherical configuration for the outer surface of the hub and the inner surface of the outer ring will reduce the cocking stiffness of the coupling and make it easier for the coupling to accommodate larger angular misalignments between the shafts that are connected by the coupling. Examples of elastomeric ring type couplings in which the outer surface of the hub is spherical in configuration are shown in FIGS. 2, 3, 6, and 7 of the previously mentioned patent to Sampson. An elastomeric ring type coupling in which both the outer surface of the hub element and the inner surface of the outer ring element are generally spherical in configuration is described and illustrated in Julien U.S. Pat. No. 2,312,470.
To provide both a high radial stiffness and a relatively low cocking stiffness in an elastomeric ring type coupling, the coupling can be fabricated with both shims and spherically shaped surfaces. Such a coupling is described and illustrated in Eksergian U.S. Pat. No. 1,868,818. The Eksergian coupling will readily permit angular misalignment between the shafts that it is interconnecting, will have a relatively high radial stiffness, and will accommodate some relative axial movement between the shafts. (The axial motion accommodation afforded by the Eksergian coupling will not, however, be as large as the motion accommodation afforded by a coupling in which the surfaces of the rigid and/or inextensible components are cylindrical). Nonetheless, because power or torque is transmitted through a ring type elastomeric coupling by placing torsional shear loads on the elastomer of the coupling, there are limitations on the amount of power or torque that can be transmitted through the coupling. To retain many of the advantages of a drive coupling such as that shown in the Eksergian patent, while also affording higher power or torque transmission capabilities, there have been developed spirally wound elastomeric couplings such as those described and illustrated in Orain U.S. Pat. No. 2,995,907.
In the elastomeric couplings of the Orain patent, the shims or laminations of inextensible material are not disposed in a concentric array between the hub and the outer ring of each coupling. Instead, the inextensible laminations are formed as long straps that are each attached at one end to the hub and are helically wound about the hub in an open spiral. The outer end of each strap is attached to the outer ring of the coupling. The elastomer or other resilient material of the coupling is interposed between the windings of the inextensible straps or laminations. The result is a drive coupling in which power or torque is transmitted through tension loads on the inextensible material of the straps or laminations and through compression loads on the elastomer interposed between successive turns or windings of the inextensible laminations. The flexing portion or section of such a spirally or helically wound drive coupling may be shaped or fabricated in much the same manner as the flexing portion of an elastomeric ring type coupling that incorporates concentric shims or laminations. Thus, as illustrated in FIGS. 1 and 3 of the Orain patent, the outer surface of the hub, the inner surface of the outer ring, and the surfaces of the spirally wound, inextensible laminations of a spirally wound coupling may all be formed with spherical configurations to provide relatively soft deflection capability through relatively large angles in response to angular misalignments of the shafts connected by the coupling.
The use of spherical shaped inextensible elements in a drive coupling that incorporates a helically wound laminated flexing section poses severe manufacturing problems. Thus, for example, it is both difficult and expensive to fabricate a long shim or strap of an inextensible material, such as a metal, in a manner such that the strap can both be wound up in a spiral and simultaneously provide an arcuate surface across its width. It is far easier and less expensive to fabricate a helically wound flexible coupling in which the surfaces of the hub, of the outer ring, and of the inextensible laminations are all cylindrical, as illustrated in FIG. 13 of the Orain patent. To have the benefits of an easier and less expensive manufacturing process, while still providing an acceptable angular misalignment capability for a helically wound coupling, it is possible to taper the widths of the laminations of inextensible and elastomeric material in the flexing section of such a coupling with increasing radial distance from the hub of the coupling. In other words, the distance between opposite sides of the flexing section, as measured in a direction parallel to the longitudinal axis of the hub and of the coupling, will progressively diminish with increasing radial distance away from the hub, as is also shown in FIG. 13 of the Orain patent. Such a coupling construction will provide increased angular misalignment capability, as compared to a similar coupling in which the widths of the laminations are not tapered, without resorting to spherically shaped surfaces. Increasing the degree of taper will increase the angular misalignment capability and decrease the cocking stiffness of the coupling and can be continued up to the point where the stresses in the inextensible laminations become unacceptably high.